1. Field of the Invention
The present invention relates to a method for fabricating a laminate film, and in particular, relates to a method for fabricating a display device (e.g. liquid crystal display device) using a laminate film and having improved viewing angle characteristic.
2. Description of Related Art
Liquid crystal display devices, which typify flat panel displays, have features of being lightweight, thin, and low in power consumption, compared with CRTs, and thus find applications in wide-range fields such as OA apparatus, car-mounted TV sets, car navigation systems, and monitors for video cameras.
A major problem relating to such liquid crystal display devices is that the viewing angle dependence is large. The viewing angle dependence refers to the following phenomenon, for example. When the screen of a display device is viewed from a direction tilting by an angle exceeding a certain angle range, an image that should correctly be displayed in black appears whitish, or reversal in gray scale levels is observed, causing reduction in display quality. From theses viewing directions, the viewer fails to correctly recognize the display image. When the angle range within which the viewer correctly recognizes display image is narrow, it is said that the viewing angle dependence is large.
The viewing angle dependence occurs for various reasons. These include, for example, twist orientation (i.e. helical structure) of liquid crystal molecules (the direction of a helix, the position at which liquid crystal molecules start forming a helix defined by the rubbing direction), the refractive index anisotropy of liquid crystal molecules (difference in retardation in the direction of propagation of light), the characteristics of a polarizing plate (whether or not the selectivity in the light oscillating direction is good), and the directivity of light rays from an area light source.
In general, transmission type liquid crystal display devices are designed, in consideration of the viewing angle dependence described above, so that the position at which the display can be viewed most nicely falls within the range in which the viewer normally views the display. For example, design is made so as to enhance the contrast ratio of the center area in the screen in the direction normal to the plane of the screen or in a somewhat downward direction from the viewer, compared with the surrounding areas in the screen.
By the above construction, however, the viewing angle range is still insufficient. In particular, liquid crystal display devices have large viewing angle dependence in the upward and downward directions with respect to the screen. In order to solve this problem, various methods have conventionally been proposed.
For example, Japanese Laid-Open Patent Publication No. 7-43703 discloses a liquid crystal display device in which a material is filled between a microlens array sheet and a liquid crystal display element. The material has a refractive index equal to or less than the smaller one of the refractive indices of the materials constituting the micro-lens array sheet and the liquid crystal display element.
Japanese Laid-Open Patent Publication No. 10-73808 discloses a liquid crystal display device in which a light diffusing sheet is placed on the front surface of a liquid crystal display element. The light diffusing sheet includes a first diffusion layer containing a light diffusing agent formed on a transparent member and a second diffusion layer having concave and convex portions formed on the first diffusion layer.
In both the above conventional techniques, the microlens array sheet is placed on a polarizing plate constituting the liquid crystal display element. The material having a refractive index which is different from that of the microlense array sheet is provided between the microlenses array sheet and the polarizing plate.
Japanese Laid-Open Patent Publication No. 7-120743 discloses a liquid crystal display device in which convex tip portions of a micro-lens array sheet are in close contact with the surface of a liquid crystal display element.
Japanese Laid-Open Patent Publication No. 9-127309 discloses a liquid crystal display device in which an adhesive layer is formed on convex tip portions of a micro-lens sheet. The ratio of the height A of the convex portion to the thickness B of the adhesive layer (A/B) must be more than 1 and equal to or less than 1000.
Japanese Laid-Open Patent Publication No. 9-194799 discloses a liquid crystal display device in which spacers are placed between a rough surface and an adhesive layer.
In the above conventional techniques, the convex tip portions of the microlens array sheet are partly put in contact with the liquid crystal display element via an adhesive layer, to control the proportion of the contact portion of the lens array to the non-contact portion thereof. In this way, the degrees of transmission and divergence of outgoing light are controlled, and thus the viewing angle characteristic is improved. In either case, the microlens array sheet is placed on the side of the liquid crystal display element closer to the viewer (viewer's side), so that light outgoing from the liquid crystal display element is diffused to the side on which the microlenses are formed (lens formation directions), to attain improvement in viewing angle characteristic.
The above conventional techniques have the following problems.
In general, a pair of polarizing plates are placed on the front and rear surfaces of a liquid crystal display element for controlling the polarizing state before display of images.
The polarizing plates are made of polyvinyl alcohol (PVA) and triacetyl cellulose (TAC). PVA is impregnated with iodine, and the resultant material is drawn in one direction to align the iodine molecules, so that light polarized along the drawn direction is absorbed (or transmitted) and thus the polarizing state of incident light can be aligned in uniform.
During the above drawing, as shown in FIGS. 22A and 22B, fine waves 222 are generated on the surface of a polarizing plate 221 along an absorption axis (or a transmission axis) as the drawn direction. This is due to a miniscule variation in the thickness of the polarizing plate 221 caused by the drawing. These waves do not influence the display when they are observed only through the polarizing plate 221. However, as shown in FIG. 23B, when a light diverging element 235 such as a microlens array sheet is placed on a surface of a polarizing plate 231, in particular, when the light diverging element 235 is bonded to a polarizing plate 231 via an adhesive layer 234, waves 232 generated on the surface of the polarizing plate are magnified. As a result, the display quality greatly deteriorates.
The display quality also greatly deteriorates in the case of using a conventional double-sided adhesive tape as the adhesive layer 234 for bonding with the light diffusing element 235 and the case of using a curable resin as the adhesive layer 234. In these cases, the contact area between the light diffusing element 235 and the adhesive layer 234 is partly changed due to scars formed by hitting with foreign substances generated in the bonding process (concave and convex deformation caused by foreign substances) and deformation caused by external force (by the viewer who touches the lens surface). This partial change in the contact area causes generation of spot defects 233a and bar-shaped defects 233b as shown in FIG. 23A.
No means for solving the above problems have been mentioned in the prior art literature.
An object of the present invention is to provide a laminate film which enables an optical film to bond uniformly to a surface (e.g., surface of a display element), even if the surface has unevenness, a method for fabricating such a laminate film, a display device using such a laminate film, and a method for fabricating such a display device.